1. Field of the Invention
This invention relates to tape drive data storage systems. More particularly, the invention is directed to thin film tape heads for reading and writing data on magnetic recording tape.
2. Description of the Prior Art
Thin film tape heads for magnetic information storage systems (e.g., tape drives) have been constructed using the same fabrication techniques used by disk drive manufacturers. A characteristic of such construction is that the thin film layers which comprise the read and write transducer elements are oriented perpendicularly to the tape bearing surface (TBS) of the head. Such heads may be referred to as “vertical” heads due to the fact that the read and write gap portions are situated at the TBS, while the element layer structures extend vertically away from the TBS. In a vertical head with multitrack recording capability, plural transducer elements are commonly arranged side-by-side to form a linear transducer array that is transverse to the direction of tape movement. Each transducer element in the array is positioned to write or read a separate longitudinal track on the tape. This arrangement is shown in FIG. 1, which depicts a vertical head “H” having an array of thin film transducer elements “E” whose gaps “G” engage a tape “T” along tracks “TR” in the direction of tape movement “D.” FIG. 2 illustrates an exemplary internal construction of the vertical head “H” in which the transducers comprise alternating read and write elements “R” and “W.” As shown in FIG. 3, the vertical head “H” of FIG. 2 can be secured to a mounting block “MB” in association with a complimentary vertical head “H” whose read and write elements are in reverse order. The resultant head assembly will have read/write element pairs that are aligned in the trackwise direction of the tape “T.” This arrangement provides conventional read-after-write capability in which data written to the tape “T” is immediately read back and checked for errors. Read-after-write capability may also be achieved with a single vertical head having pairs of trackwise-aligned read and write elements that are constructed according to the well-known “piggyback” arrangement used in disk drives. Other conventional vertical head designs include heads in which all of the transducer elements “E” are either read elements or write elements. Read-after-write capability may then be achieved by bonding a read-only head to a write-only head to provide trackwise-aligned read and write element pairs.
A disadvantage of vertical head constructions as described above is that the transducer element gaps at the TBS must be sufficiently spaced from each other to provide room for the major portion of the transducer element structure that is recessed behind the TBS. For a write element, the recessed structure includes the pole pieces and the coil windings, which (as can be seen in FIG. 2) are quite bulky as compared to the write gap structure at the TBS. For a read element, the recessed structure includes the electrical leads and magnetic hard biasing elements (if present). These are also relatively bulky compared to the read gap structure at the TBS, although less so compared to write elements. The foregoing spacing requirements render the transducer array of a vertical head much wider than it needs to be for the number of tracks being read or written at any given moment. The problem is that the gap pitch within the transducer array is much larger than the gap width, such that for every track being read or written by the array, there will be space between the tracks where no transducing occurs. This “comb” effect can be seen in FIG. 2, which shows that for every pair of tracks “TR” aligned with adjacent read and write elements “R” and “W,” there is inter-track white space on the tape “T” that is not tranduced.
The comb effect can be solved by stepping the head in a cross-track direction during multiple transducing passes, such that the inter-track white space is ultimately recorded with data after some number of passes have been made. Tape tracks can also be written at less than the gap width of the write transducers using a process known as “shingling.” According to this technique, the head is stepped by less than the write element gap width for each successive transducing pass, such that the edge of a previously written track is overwritten during the next pass, much like shingles on a roof.
Although the foregoing track writing techniques allow data to be densely packed on a tape, a continuing unresolved problem is track misregistration caused by tape dimensional changes between writing and reading operations. For example, a tape may be written with data under one set of temperature and humidity conditions, and then later read following exposure to different environmental conditions. For conventional tape material, the dimensions can change by as much as 0.12%. These tape dimensional changes will widen or narrow the tape track spacing geometry, resulting in track misregistration with the tape head whose gap spacing geometry is substantially unchanged. Although rotation of the tape head can be used to address the misregistration problem by changing the effective track pitch of the transducer array, this solution requires sophisticated mechanics and skew compensation circuitry.
To illustrate the misregistration problem, assume the transducer array spans×μm between the outermost elements, and the percentage change in tape dimension is 0.12%. The resultant change in the spacing of the tape tracks under the outermost elements will be 0.0012×μm. On the other hand, if the transducer array spans 0.5×μm, then a 0.12% change in tape dimension will only change the tape track spacing under the outermost elements by 0.0006×μm. The 0.5× transducer array span will thus experience only half of the tape dimensional change that is experienced by the x transducer span, such that track misregistration is less likely. Note that if the head gap pitch can be reduced to a value which approaches the gap width, a further advantage of being able to perform contiguous-track bundle writing could be achieved. The head would then be able to read and write the entire tape area underlying the transducer array in a single pass, thereby alleviating the precise tracking required for shingling tracks in multiple passes.
Accordingly, it is desired to have an improved design for a thin film tape head for reading and writing data on magnetic recording tape. What is particularly needed is a head design that provides the ability to reduce the gap pitch of read and write elements, especially to levels approaching the gap width.